am57x: Move CONS_INDEX to Kconfig

- Move the CONS_INDEX selection out of CONFIG_SYS_EXTRA_OPTIONS and
      into Kconfig proper.
    - Edit the relevant am57x configs to remove the now unneeded
      CONFIG_SYS_EXTRA_OPTIONS.

Signed-off-by: Daniel Allred <d-allred@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/board/ti/am57xx/Kconfig b/board/ti/am57xx/Kconfig
index 17745ff..87654f9 100644
--- a/board/ti/am57xx/Kconfig
+++ b/board/ti/am57xx/Kconfig
@@ -9,6 +9,15 @@
 config SYS_CONFIG_NAME
 	default "am57xx_evm"
 
+config CONS_INDEX
+        int "UART used for console"
+        range 1 6
+        default 3
+        help
+          The AM57x (and DRA7xx) SoC has a total of 6 UARTs available to it.
+          Depending on your specific board you may want something other than UART3
+          here.
+
 source "board/ti/common/Kconfig"
 
 endif